1. Field of the Invention
The present invention relates to an optical head and an optical disk apparatus, each designed to record and reproduce data on and reproducing data from optical disks. More particularly, the invention relates to an optical head that can detect the thickness of the transparent substrate of any optical disk and an optical disk apparatus that comprises such an optical head.
2. Description of the Related Art
As well known in the art, optical disks comprise a transparent substrate and a hard protective layer. The interface between the substrate and the protective layer serves as a recording surface. A focused light beam emitted from an optical head may be applied to an optical disk. The light beam passes through the transparent substrate, reaching the recording surface. Data is thereby recorded on, or read from, the recording surface.
Generally, the transparent substrates of optical disks fail to have a design thickness, because they have been formed in different manufacturing conditions. The difference between the thickness of the substrate and the design thickness (hereinafter referred to as “thickness error”) is about tens of microns (μm) in most cases. The thickness error results in spherical aberration. Due to the spherical aberration, the light spots the light beams passing though transparent substrates, which differ in thickness, form on the recording surfaces differ in shape.
This not only decreases the density of recording data on the recording surface, but also degrades the reliability of recording data thereon. Further, this degrades the reliability of reproducing data from the recording surface. It is inevitably difficult to record and reproduce data reliably and stably, on and from the optical disk.
To record and reproduce data reliably and stably, on and from an optical disk, the optical head needs to decrease the spherical aberration in the optical disk to a tolerant value. To change the spherical aberration appropriately, the spherical aberration must be determined with precision. For precise determination of the spherical aberration, it is necessary to measure the thickness of the transparent substrate.
Jpn. Pat. Appln. KOKAI Publication No. 2000-76665, for example, discloses a system for determining the thickness of a transparent substrate. The system comprises means comprises light-emitting means for emitting a light beam to the transparent substrate and light-receiving means for receiving the light beam reflected from the substrate. Both the light-emitting means and the light-receiving means are incorporated in an optical head.
The system may further comprise an objective lens, the center part of which differs in curvature from the peripheral part. Of the light beam passing through the objective lens, only the part that has passed through the center part is used to determine the thickness of the transparent substrate.
Alternatively, the system may further comprise a hologram element arranged on the optical path of the optical head. In this case, the thickness of the transparent substrate is determined from the primary refracted light emanating from the hologram element. The light reflected from the recording surface and the surface of the transparent substrate is led to a hologram. The hologram splits the light into two light beams. A photodetector detects the light beams. It and generates a focusing error signal from one of the light beams, or the light reflected from the recording surface. It also generates a signal from the other of the light beams, the light reflected from the surface of the transparent substrate. The two signals, thus generated, are subjected to an arithmetic operation. More precisely, the magnitude of the focusing error signal generated is subtracted from a product of a prescribed proportion coefficient and the signal generated from the light reflected from the surface of the transparent substrate. The difference obtained is a signal that represents the thickness of the transparent substrate.
In the system comprising an objective lens, however, a light beam applied to the recording surface travels in one optical path, and the light beam reflected from the recording surface travels in another optical path. This renders it difficult to split a signal and, ultimately, to generate a signal that accurately represents the thickness of the transparent substrate. To make the matter worse, the range over which the thickness error of the transparent substrate can be detected is inevitably the same as the range over which the focusing error can be detected. This is inevitably because the hologram element spits the light reflected from the recording surface and the surface of the transparent substrate.
In the system comprising a hologram element that can diffract light, the primary refracted light emanating from the hologram element arranged on the optical path of the optical head is used to determine the thickness of the transparent substrate. When a light beam is applied to the optical disk, 0th-order diffracted light is generated. When the light beam is reflected from the optical disk, high-order diffracted light is generated. It is therefore difficult to split a signal.
As has been pointed out, with the conventional systems for determining the thickness of the transparent substrate of an optical disk it is difficult to separate the light reflected from the recording surface of the optical disk from the light reflected from the transparent substrate. Further, the conventional system that comprises an objective lens is disadvantageous in that the range over which the thickness error of the transparent substrate can be detected is inevitably the same as the range over which the focusing error can be detected.